This invention relates generally to refrigerant compressors and, more particularly, to centrifugal compressors with a unique diffuser and collector combination for obtaining high efficiency performance.
In large capacity air conditioning systems using water-cooled chillers, centrifugal compressors are most commonly used. The refrigerant of choice in such compressors has commonly been CFC-11, which is relatively high in thermodynamic cycle efficiency.
Given the use of this common refrigerant, and given the system capacity requirements for a particular installation (i.e. the head or pressure ratio and the flow requirements), then the size of the various components can be determined. If the speed is considered to be fixed, as is generally the case, then the impeller diameter and width are chosen to fit the particular capacity requirements. It is, of course, the impeller which accelerates the refrigerant to a high velocity, after which it is necessary to decelerate the refrigerant to a low velocity while converting kinetic energy to pressure energy. This is commonly done with the diffuser and, to some extent, with the chamber into which the diffuser discharges its refrigerant.
With the use of CFC-11 as the refrigerant, it is generally understood that complete diffusion (i.e. the conversion of substantially all of the kinetic energy to pressure energy) cannot be accomplished within the normal constraints of available diffuser space. That is, such a diffuser would be inordinately large with respect to the drive motor and gear systems, and would severely detract from the practical application of such a system. The normal approach is therefore to complete the diffusion process in a spiral shaped casing called a volute. The volute therefore functions to both complete the diffusion process and to collect the discharge vapor for subsequent flow to the condenser. While the volute with its gradually increasing cross section provides an optimum design for the use of available space, it is recognized that some efficiency is lost in the diffusion that takes place in the volute.
Typical centrifugal compressors have a volute with an outside diameter which is about twice the impeller diameter. Under these geometrical conditions, the amount of diffusion in the compressor is therefore limited. To obtain further diffusion, it would not only require a larger diffuser outside diameter, and therefore a larger volute diameter, but it would also require a larger cross sectional area in the volute in order to enable the passing of a given volumetric flow rate at lower velocities. Because of these constraints, centrifugal compressors with conventional, size-limited diffuser/volute combinations experience increased circumferential flow distortions under part load conditions when the volute becomes oversized and starts acting as a circumferential diffuser. Resulting circumferential pressure buildup and its corresponding flow nonuniformities have been felt upstream of the diffuser and even at the inlet of the impeller. The effects of these nonuniformities on overall compressor performance are loss in efficiency and a reduction of stable operating range under part load conditions.
It is therefore an object of the present invention to provide an improved centrifugal compressor method and apparatus.
Another object of the present invention is the provision in a centrifugal compressor for improving the efficiency thereof while remaining within the given geometric constraints.
Yet another object of the present invention is the provision in a centrifugal compressor for avoiding the performance losses that result from circumferential flow distortions under part load conditions.
Still another object of the present invention is the provision in a centrifugal compressor for more efficiently converting the kinetic energy to pressure energy and for collecting the decelerated gas for use in the condenser.
Still another object of the present invention is the provision for a centrifugal compressor which is economical to manufacture and efficient and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.